Resolution measurement of a digital camera is usually accomplished by capturing an ISO chart, printing or displaying the captured image, and then visually evaluating the image in a subjective way to obtain a visually measured resolution value or using a software measurement method consistent with the visual evaluation method to evaluate image to acquire the resolution value.
In the visual evaluation method by visual inspection, an ISO pattern is printed or magnified by a fixed factor to observe a spatial frequency at which wedge-shaped lines vary, and the spatial frequency is taken as the resolution. Although the visual evaluation method is simple, it has shortcomings of being liable to influence of individual differences, time and labor consuming, and limited by the displaying and printing devices. The software measurement method can obtain a result as good as the visual evaluation method and is not limited by the devices and the labor, so it has been widely used.
To clearly obtain the resolution data, conventionally a testing method based on the Modulation Transfer Function (MTF) is most commonly used. The MTF mainly measures the contrast M=(Imax−Imin)/(Imax+Imin) to reflect the resolving power of human eyes, where Imax and Imin are the maximum value and the minimum value of illumination respectively. Additionally, the Spatial Frequency Response (SFR) measurement has become another means that is commonly used in recent years. The SFR measurement mainly measures the influence of increasing spatial frequency lines for a single image.
The visual evaluation method is simple to operate but is limited by the devices and consumes labor, so it is unsuitable for the wide automatic industrial production. The MTF-based measurement method (e.g., the HYRes software from Japan) has long been proved to be effective, but with the advent of digital cameras of the 10-million pixel scale, the capturing resolution has already exceeded the limits of USAF1951 and ISO12233, which makes the MTF-based measurement method unable to test such digital cameras. Meanwhile, the MTF-based measurement method is computation intensive, and does not allow for real-time dynamic processing. The SFR-based measurement method (e.g., the Imatest software from the U.S.) has a low cost and provides a good drawing effect, but it is complex to operate, demands higher skills of the testing personnel, and does not allow for real-time dynamic testing.